The step selector for a step transformer comprises a cylindrical drum made of insulating material and normally centered on an upright axis. Mounted in the drum are annular arrays of contacts for preselecting, changeover, and fine selecting. Coaxial shafts extending along the axis in the drum carry arms having terminals that are brought into engagement with these contacts to select the desired transformer steps.
During switching by a coarse selector of changeover device the step windings of a step transformer are briefly separated galvanically from the main winding. During this time these windings develop voltages, induced from adjacent windings as well as capacitatively with respect to these windings and to ground. Since these voltages can become substantial, it is therefore standard to provide a system which can relieve them to avoid damage to the switching terminals and contacts.
This is done by a polarity switch which maintains the step windings at a predetermined potential by connecting them together through a resistance during the brief switchover time. Such a polarity switch is described in German patent 1,942,567 filed 21 Aug. 1969 by W. Weber et al and in German patent document 2,548,408 filed 23 Oct. 1975 by U. Schweitzer et al. It is coaxial with the preselecting and fine-selecting contact systems and below the step selector, substantially increasing the height of the system. In German patent 2,815,736 filed 12 Apr. 1978 by A. Bleibtreu et al the polarity switch is mounted on the side of the step-selector drum. This increases the width of the system and is fairly complex. Both systems require a separate drive for the polarity switch, increasing equipment costs and the possibility of parts failure.
Other systems are described in German patent document 1,103,434 filed 2 Mar. 1960 by O. Leopold and in German patent 3,719,680 filed with a claim to an Austrian priority date of 02 Jul. 1986 by H. Schmidt. These systems also are fairly bulky and entail substantial extra equipment.